This invention relates to an inventer-driven power supply system which transfers electric power from a d.c. power source to a load in parallel connection with a capacitor through an inductive circuit element and, particularly, to a power supply system which is capable of settling the load application voltage to the specified voltage without delay at the start-up of power supply and suppressing the oscillation of the application voltage.
There has been known this type of inverter system made up in serial connection of a d.c. power source 1 having terminal voltage E, a switch 2, a circuit element, e.g. a winding 3 having inductance L, and a load 5 having conductance G in parallel connection with a capacitor 4 having capacitance C, as shown in FIG. 1. The serial circuit includes a resistive component R for the wiring. In the above circuit arrangement, when the switch 2 is closed at time point t.sub.o as shown in FIG. 3, to supply power to the load 5, the current i flows along the route from 1 to 2 to 3 to R to 4 and 5 in parallel, and back to 1, and power supply to the load 5 commences. At this time, the application voltage V to the load 5 rises as shown by the solid line 6 in FIG. 3B and the voltage is often oscillatory during the transitional period until the current i settles to the stationary state. There are three cases of transient response for the circuit shown in FIG. 1 depending on the values of the inductance L, resistance R, capacitance C and conductance G, as follows.
(1) Aperiodic damping response, in case of: ##EQU1## (2) Critical damping response, in case of: ##EQU2## (3) Oscillatory response, in case of: ##EQU3##
For the inverter system shown in FIG. 1, the resistance R is made as small as possible, in general, so as to achieve a high power transmission efficiency. On this account the voltage is apt to oscillate as categorized in case (3). IN order for the conventional power supply system to suppress the oscillation of the application voltage to the load 5, it includes a resistance R' large enough to suppress the voltage oscillation in series to the resistance R when the switch 2 is closed to commence power supply as shown in FIG. 2 and then shunts the resistance R' after the voltage has settled to the stationary state.
Although this power supply system is capable of suppressing the oscillation of application voltage V to the load 5 at the commencement of power supply by the effect of the resistance R' as shown by the dash-dot line in FIG. 3B, voltage rise time until the application voltage V reaches a certain voltage level, e.g., the terminal voltage E of the d.c. power source 1, is delayed significantly. Therefore, the system is not applicable to some types of load 5 where the allowable voltage rise time t is restricted.
Next, the conventional power supply system with its load being an X-ray generator will be described with reference to FIGS. 4 to 7. A typical conventional X-ray generator receives the commercial a.c. power voltage, and the voltage is adjusted in such a way of selecting the position of the slide brush on the secondary winding of a voltage control transformer, raised by a step-up transformer, rectified into a d.c. voltage, and applied to an X-ray tube.
Recently, inverter-driven X-ray generators have been developed by utilization of the advanced power control semiconductor devices. Owing to the use of such power control semiconductor devices, the inverter-driven X-ray generator is incomparably faster in power control than the first-mentioned one using a voltage control transformer, allowing the easy and accurate tube voltage adjustment during the X-ray emission.
FIG. 4 shows the circuit arrngement of the conventional inverter-driven X-ray generator. The system includes a d.c. power source 31 providing input power to the inverter, transistors 32-35 each becoming conductive in response to the base current so as to convert the d.c. voltage into an a.c. voltage through the alternating states of conductive transistors 33 and 34 and conductive transistors 32 and 35, and diodes 36-39 connected in antiparallel fashion to the respective transistors 32-35 for the purpose of retrieving energy held in the circuit back to the d.c. power source 31, the components 32-39 in combination constituting an inverter. The system further includes a step-up transformer 40 for raising the output voltage of the inverter, diodes 41-44 constituting a full-bridge inverter circuit, and a capacitive component 45 which actually exists distributively on a high-tension cable connecting the rectifier output to an X-ray tube 46.
The step-up transformer 40 has a large turn ratio, with the secondary winding being placed on multiple layers, which allows the stray capacitance to exist between the layers as shown by the equivalent circuit of the transformer 40 in FIG. 5A. The equivalent circuit is further simplified as shown in FIG. 5B. The transformer is equivalently expressed by itself as a combination of leakage inductances L.sub.l1 and L.sub.l2 and an exciting inductance L.sub.ex, and the transformer with the stray capacitance Cs on its secondary winding has the equivalent circuit shown in FIG. 5C. Under the evident conditions L.sub.l1 &lt;&lt;L.sub.ex and L.sub.l2 &lt;&lt;L.sub.e x and on the assumption L.sub.l =L.sub.l 1+L.sub.l2, the equivalent circuit of the step-up transformer 40 is simplified as shown in FIG. 5D.
FIG. 6 shows the inverter-driven X-ray generator derived from FIG. 4, but with the step-up transformer 40 being replaced with its equivalent circuit including the leakage inductance L.sub.l and stray capacitance C.sub.s shown in FIG. 5D. The components 51-59 and 61-66 are all counterparts of FIG. 4.
The operation of the system shown in FIG. 6 will be described by making reference to the waveform diagram of FIG. 7. When the transistors 52 and 55 are made conductive by the base currents a and d, respectively, the load current i.sub.I flows through the route from 51 to 52 to L.sub.l to 61 to 65 and 66 in parallel, to 64 to 55, and back to 51, and power is supplied to the load. At this time, the stray capacitance C.sub.s is charged in polarity as shown in FIG. 6. When the base currents a and d cease at time point t.sub.o in FIG. 7, causing the transistors 52 and 55 to cut off, the load current i.sub.I falls to zero. Following a halt period which is provided so that the transistors 53 and 55 (or 52 and 54) are not conductive simultaneously, the base currents b and c are supplied at time point t.sub.1 to make the transistors 53 and 54 conductive. During the period from t.sub.o to t.sub.1 the stray capacitance C.sub.s retains the polarity of charge as shown in FIG. 6. Accordingly, when the transistors 53 and 54 become conductive at t.sub.1, the current flows through the route from 51 to 53 to C.sub.s to L.sub.l to 54, and back to 1 as shown by the dashed line in FIG. 6, and the stray capacitance C.sub.s is charged reversely. This reverse charging takes place in a resonance circuit including the leakage inductance L.sub.l and stray capacitance C.sub.s which are energized by the voltage of the d.c. power source 51 plus the voltage of the stray capacitance C.sub.s, causing the creation of an excessive oscillatory current. Therefore, the load current i.sub.I becomes oscillatory as shown in FIG. 7. The oscillating currett has a peak value i.sub.IP, which is expressed in terms of the voltage E of the d.c. power source 51 and the voltage V of the stray capacitance C.sub.s as follows. ##EQU4## Since the voltage V is nearly equal to E at time point t.sub.o, the above equation is reduced to at this time point as follows. ##EQU5## The voltage V of the stray capacitance C.sub.s reaches the maximum at time point t.sub.2 when the first half-cycle of the oscillating current completes, and it is higher than the voltage of the d.c. power source 51. The voltage V.sub.x applied to the X-ray tube 66 and the elecrostatic capacity 65 existing on the high tension cable is equal to the voltage V of the stray capacity C.sub.s, and it is maximum at time point t.sub.2. The voltage V.sub.x created by the oscillation of current in the leakage inductance L.sub.l and stray capacitance C.sub.s is in excess of the voltage which would be raised from the voltage of the d.c. power source 51 by the turn ratio of the step-up transformer 40. Thereafter, the stray capacitance C.sub.s is discharged through the route from C.sub.s to 57 to 51 to 58 to L.sub.l, and back to C.sub.s, and power is returned to the power source instead of being supplied to the load. The only power supply to the X-ray tube 66 owes to the discharging of the electrostatic capacity 65 existing on the high tension cable, and the voltage V.sub.x falls as shown in FIG. 7. The oscillation of the load current diminishes progressively, and the stray capacitance C.sub.s is charged in polarity opposite to that shown in FIG. 6 at time point t.sub.3 when the transistors 53 and 54 are cut off. An operating cycle completes, and the same cycle takes place iteratively.
The stray capacitance of the step-up transformer is charged each time the polarity of the inverter output voltage reverses, with the polarity of charging being consistent to the output voltage polarity. The charging current becomes an oscillating current due to the leakage inductance of the step-up transformer, resulting in an enlarged pulsation on the tube voltage waveform in excess of the inverter output voltage multiplied by the step-up ratio of the transformer.
The current flowing through the transistors 52-55 is the load current added by the oscillating current, and therefore these switching devices need to be chosen in consideration of the peak current. It is required that the oscillating current caused by the leakage inductance and stray capacitance of the step-up transformer does not adversely affect the tube voltage waveform.
The prevention of load current oscillation caused by the leakage inductance of the step-up transformer and the electrostatic capacity of the high tension cable in the inverter-driven X-ray generator as described above is disclosed in Japanese Patent Unexamined Publication No. 57-53100, in which a resistor is inserted in the circuit so as to provide a damping effect.